1. Field of the Invention
The disclosure herein relates generally to the field of electromagnetic transducers. More specifically, the present disclosure relates to an electromagnetic acoustic transducer used in non-destructive testing. Yet more specifically, described herein is a method and apparatus for eliminating interference between separate electromagnetic acoustic transducers.
2. Description of Related Art
Monitoring the behavior of acoustic waves in a solid is useful in detecting potential flaws in the solid. One example of use includes propagating an acoustic wave into a member being testing, receiving the resulting wave, and analyzing the wave. Determining the resulting wave's attenuation can yield useful information concerning flaws in the member. The flaws may include cracks, pitting, corrosion, or other discontinuities in the solid. The members being tested include structural members, vessels, piping and other tubulars. Other applications include measuring solid dimensions and identifying the material through which the wave propagates.
One device useful for inducing acoustic waves in solids for non-destructive testing is an electromagnetic acoustic transducer (EMAT). FIG. 1 illustrates in a side cut-away view an example of a prior art EMAT 10. The EMAT 10 comprises a permanent magnet 14 that extends substantially parallel to an electrically conductive object 12. Members disposed on the terminal ends of the magnet 14 form a magnetic yoke 16 extending downward toward the object 12. A coil 18 comprising a series of wires 20 is disposed in the space between the permanent magnet 14 and the object 12.
EMAT function comprises flowing electrical current through the coil 18 thereby inducing eddy currents in the object 12 proximate to the electrically conducting wire 20. Interaction between a magnetic field and induced eddy currents in turn creates Lorentz forces that acoustically excite the object. The magnetic field is produced by the magnet 14. Acoustic excitation typically results in acoustic waves that propagate in the object 12. Similarly, placing an EMAT proximate to an object excited by acoustic waves can induce an alternating magnetic flux that in turn results in an electromotive force applied to the receiver coil wires. Thus by measuring this electromotive force an EMAT may also act as an acoustic receiver. Recording and analyzing these waves is useful in detecting flaws in the solid.
One drawback of currently used EMATs is if an EMAT transmitter and an EMAT receiver are sufficiently proximate on another, parasitic coupling, or cross-talk, occurs between the respective windings of the transmitter and receiver. The resulting cross talk can have deleterious effects on data received by the receiver.